This invention relates generally to satellite-based communication systems, and more particularly to a satellite-based communication system having multiple satellites without requiring satellite crosslinks.
Satellite-based communication systems which provide wireless access between separated user terminals on the earth are designed to give users of laptop PCs, handheld computer, and other similar devices very high-speed two-way data links with the Internet, their corporate Intranets, each other, and any other desired network. The service coverage is international, and utilizes high-power satellite transponders that route data from one user terminal to another.
The satellite-based systems presently under consideration include at least two different types of satellite constellations. One type, such as that described as the TELEDESIC(trademark) or ICO Global systems, provides a plurality of satellites in low-earth or medium-earth orbits, in which satellites enter and exit from view of a user terminal and require handoffs from one satellite to another as the individual satellites traverse into and out of view of a particular user. These handoffs typically occur via intersatellite crosslinks which route incoming data from a user from one satellite to the next. These intersatellite crosslinks are expensive to manufacture, are heavy and thus increase launch costs, take up valuable real estate on the satellite, and consume precious power.
A second type of system, such as that known as ASTROLINK(trademark), uses satellites in geostationary or geosynchronous orbit (GEO), which essentially is viewed as stationary in the sky by a user on the earth. Satellite handoffs are not necessary in the basic system, since only one satellite is ever in the field of view of the user.
However, in order to provide redundancy and to provide increased capacity, certain GEO systems may utilize two or more satellites in one node along the geostationary arc. This use of multiple satellites at one node can cause a problem, in that the ground terminals transmit and receive to and from only one satellite at a time. Communications between users accessing different satellites must be routed either on inter-satellite crosslinks or through a ground-bounce gateway so that user data is routed to the satellite to which the user is connected. Users within a particular geographic region will be divided between the two satellites in the node. As a result, statistically, approximately half the data within a region will be routed between satellites. However, the ground-bounce gateway is an inefficient utilization of system resources. It therefore is desirable to have a system which can interconnect user terminals within a geographic region without requiring either intersatellite crosslinks or a double hop through a ground gateway.
In accordance with the present invention, the aforementioned need in the prior art is met by a communications system which includes two or more satellites each including means for receiving RF signals through an uplink channel and for transmitting RF signals through a downlink channel distinct from said first uplink channel. At least two user terminals are in the system, with each user terminal able to transmit RF signals to only one of the satellites, but both user terminals able to receive RF signals from both of the satellites.
An alternate embodiment also has two user terminals, but only one of the terminals has the capability to receive RF signals from both satellites. The other user terminal can receive signals from only one of the satellites. The first receiving satellite, in this instance, has the ability to determine the receiving capability of the second user terminal and, if the signal for this second terminal is one that the user terminal cannot receive directly, the satellite can route the signal through a ground gateway to the other satellite so that the transmission can be completed.